Mudslide kills three, leaves 18 unaccounted for in US north-west

Emergency services personnel are searching for those missing since the mudslide.

Washington State Police

Three people have died and 18 are missing after a massive landslide slammed into a mountainside community in the United States north-west.

A six-month-old infant was among eight people injured when mud, water and rock smashed into the rural town of Oso, north-east of Seattle, police and firefighters said.

Six homes and much of a highway were destroyed.

"There is a full-scale, 100 per cent, aggressive rescue effort," Washington state governor Jay Inslee said, adding that helicopters, hovercrafts and rescue personnel had been sent to the scene.

"There's no missing piece in this rescue effort."

A state of emergency has been declared.

Snohomish County spokeswoman Shari Ireton said they "are able to confirm ... that 18 people who may have been in the area are unaccounted for. We do not have identification for those people that we are releasing at this time".

The search and salvage operation in the mud-covered terrain was extremely dangerous with the unstable, rain-saturated surface putting emergency personnel at risk of further mudslides.

"Some (rescue workers) went in and got caught literally up to their armpits" in mud and had to be pulled out themselves, Mr Inslee said.